In machine processing areas having a volatile atmosphere, such as film fabrication, means of venting in the event of a deflagration is generally provided, thus preventing damage to the machine itself. Fabrication of solvent based film involves casting a liquid dopant onto a polished surface, such as a stainless steel band or a wheel to produce the film. The film is then stripped from the polished surface in order to remove the solvents from the film prior to winding the film in roll form. To prevent solvent emissions and to permit recovery of the solvent, the equipment is contained within a sealed, insulating casing. Explosion panels are generally provided to relieve the pressure with the casing in the event of an explosion. Research Disclosure No. 35518, dated November 1993, describes one such panel constructed of a frame incorporating several layers of a clear film. The layers of a film such as polyester are used to provide thermal insulation and reduce inertia of the panel. The panels are attached to the casing or enclosure by capturing the periphery of the frame at one of the layers of film between an annular metal frame surrounding the opening of the casing and a continuous annular magnet. When a pressure wave occurs due to an explosion, the periphery pulls out from between the magnet and the frame surrounding the opening of the casing and the panel is pushed outside of the casing. Low pressure (1/2 to 3/4 psi) release venting is achievable with the above described panel.
U.S. Pat. No. 4,777,974, issued Oct. 18, 1988 to Swift et al., discloses a pressure relief panel assembly for rupture disks, explosion vents and the like characterized by predetermined weaknesses in a plurality of panel portions which burst at different pressures.
U.S. Pat. No. 3,250,206, issued May 10, 1966 to Strouth, discloses a ventilation system for a building roof that includes a magnetic element incorporated in a frame for securement to the building.
U.S. Pat. No. 3,182,855, issued Oct. 18, 1962 to Stock, discloses a blow-out door adhesively bonded to a fluid containing shell structure for overpressure protection.
Other types of venting panels are described in, for instance, U.S. Pat. No. 4,498,261 directed to a vent panel comprising a sealing membrane bonded to the panel; and, U.S. Pat. No. 3,189,675 related to a pressure relief device for relieving sealed electrical devices from internal pressure.
Thus, means of accomplishing overpressure venting generally includes an opening in the machine enclosure sized relative to the volume of the enclosure and a cover for the opening. Under rather rigid fire regulations, promulgated by the National Fire Prevention Association (NFPA), the panel must meet specific requirements to be considered in compliance with standards, some of which are enumerated below. (See NFPA 68, entitled Guide for Venting Deflagrations, page 15, 1988 ed.). According to NFPA, the panel must be designed, constructed, installed and maintained so that it will readily release and move out of the path of the combustion gases. The panel must also not become a missile hazard when it operates.
Further, according to NFPA, the total weight of the panel assembly, including any insulation and permanently mounted hardware, should be as low as practical, but in no case should it exceed 2.5 lbs per sq. ft. The purpose of this limitation is to keep the inertia of the assembly as low as possible, so that the vent opens as rapidly as possible.
Moreover, as indicated by NFPA, the material construction of the panel should be suitable for the environment to which it will be exposed. Brittle materials will fragment, producing potentially lethal missiles. Some panels because of their configuration, may travel some distance from the enclosure. Each installation must be evaluated to determine the extent of hazard to personnel from such missiles.
Furthermore, according to the NFPA, the vent panels must release at as low an internal pressure as practical, yet stay in place when subjected to external wind forces. Also, the panel must provide the required vent area for the volume of the enclosure being protected. The panel, moreover, must not release at pressures which are normally experienced during normal operations.
Since it is required that the panel be as light weight as possible, the regulations imposing only an upper limit, many designs of vent panels employ membranes, such as aluminum or plastic sheet stock (see for example U.S. Pat. No. 4,498,261). One way known to retain the membrane to the enclosure includes the use of magnets having a holding force such that under a specific enclosure pressure the membrane is pulled from under the magnets, or the magnets separate from the machine enclosure, and free the membrane. Another known way to retain the membrane to the enclosure is by the use of a spring loaded frame. In this case, the spring force is sized so that the membrane is pulled from under frame when a specified pressure is reached. Yet another way to retain the membrane on the enclosure is by clamping it to the machine frame having serrated edges. In this case, when the overpressure in the enclosure urges the membrane onto the edges of the serration, the membrane is cut free from the enclosure.
Although specific disadvantages and advantages are associated with each of these prior art designs or configurations, the membrane type vent panel, as discussed above, has many shortcomings including poor thermal characteristics, i.e., the panel transmits heat in and out of the enclosure during normal operations, and susceptibility to causing injury. Also, the membrane type panel does not provide suitable access to the interior of the enclosure.
It is known that a rigid panel may address some of the above disadvantages, especially those related to poor insulating characteristics. However, rigid panels tend to exceed the regulations weight requirements. While some recently developed light weight plastic materials may offer new opportunities to deal with the weight problem, these materials often raise issues of chemical compatibility and distortion due to thermal expansion, i.e., changes in temperature of the enclosure during normal operations.
Other conventional means of securing such a vent panel to an enclosure having the capability of releasing the panel when a certain pressure within machine enclosure is reached, include: shear bolts or similar elements, and mechanical latches designed to release under a certain load.
The shear bolt/element method has the advantage of simplicity, but makes impractical the use of such a vent panel as a means of providing access to the interior of the enclosure. Also, the release force is dependent upon the actual strength characteristics of the material. This may vary over a wide range for a given type of material. The actual release force of this design cannot be verified.
A significant disadvantage of the mechanical design latch is that it is complex and needs to be adjusted for a specific application. The adjustment cannot be readily calibrated and is not tamper resistant. Moreover, the latch function may be affected by dirt.